The present invention relates generally to integrated circuits and, more particularly, to monitoring metal conductor resistivity for possible failure due to thermal cycling of the integrated circuit.
Voltage regulators integrated circuits are well known. Generally, a voltage regulator is used to provide a desired constant voltage over a wide range of current supplied to a load, such as a microcontroller. Each voltage regulator includes one or more power devices, such as a metal-oxide-semiconductor field-effect transistor (MOSFET) or bipolar transistor, operating as a switch or a pass transistor to couple current to the load. As a consequence of conducting current, the power device generates heat that warms the chip, the hottest portion of the chip (a “hotspot”) being proximate the power device. To protect the chip from overheating such as during an overload condition (e.g., when supplying current to an abnormal load such as a short circuit), a temperature sensor on the chip disables the power device from conducting when the chip reaches a maximum temperature. The power device remains disabled until the temperature drops below a temperature deemed safe for the chip to operate. Under worst-case conditions, the voltage regulator chip might cycle between the maximum temperature and the safe operating temperature several hundred times per second until the abnormal load is removed.
A typical voltage regulator chip will have two or more layers of metal conductors overlying the power devices and other components (e.g., resistors, non-power transistors, diodes, etc.) formed in the substrate of the chip. Each thermal cycle of the chip causes the metal conductors on the chip to expand and contract, resulting in stresses on the conductors. These stresses can, with each cycle, damage the metal conductors. The damage might be most predominant in the conductors formed in the top metal layer because those conductors are generally the widest, carry the most current, and are used to interconnect with other chips or external circuitry using interconnects such as bond wires or solder balls. The accumulated effect of the thermal cycling on the top metal conductors might result in an unacceptable and irreversible increase in the resistance of the metal conductors carrying the most current, thereby causing the chip to fail. Accordingly, it would be advantageous to have a technique for providing an indication that the chip is about to fail due to, for example, the above-described effects of thermal cycling.